First, a warning about the truth or otherwise of what you are about to read. This is an article about documentary features. Specifically, it is an article about documentary features that alert us to the fact that they are documentary features. Documentaries that lift the bonnet to show the engine. Documentaries that remind us that they are authored pieces of work as opposed to some objective, inviolate truth. Films are made by film-makers, after all, just as newspaper features are written by journalists with one eye on the deadline and the other on the tea break. They cherry-pick their sources and manipulate their material. As such, they are not entirely to be trusted.

The Arbor is a superb new film by the British artist Clio Barnard. It paints a portrait of the Bradford playwright Andrea Dunbar and then charts the tragic fallout from her death – courtesy of an alcohol-induced brain haemorrhage – at the age of 29. Barnard interviewed Dunbar's family friends and then, in an ingenious twist, hired professional actors to lip-synch over the audio track. The effect is compelling, because it draws attention to what is "real" (the audio) and what is "fake" (the acting). It makes the two forms talk to each other, as it were, to the point where we start to question the veracity of everything we are seeing and hearing.

This, it transpires, is what Barnard intended. "If you examine any documentary, you see how shaped it is, and how similar the narrative structure is to that of a fiction film," she says. "The lip-synching allows the actor to look directly down the lens at the audience: to acknowledge the illusion and break the fourth wall."

The negotiation between fiction and reality, she adds, is always tricky. "Drawing attention to that negotiation is a more honest way forward than relying on the technique to do the job and smooth out the tensions."

It also chimes with the material. In the first place, Dunbar wrote plays that were semi-autobiographical representations of her experiences on the Bradford estates. She brushed up true-life conversations and dropped them into the mouths of middle-class performers. Then there is the fact that Barnard's film is expressly concerned with false memory, with conflicting versions of the truth. One of Dunbar's daughters (Lisa) won't say a bad word against her mum. The other (Lorraine) says nothing but. All of which makes The Arbor intriguing, and gloriously perplexing, too. At this year's Tribeca film festival, where it picked up a prize, some members of the jury disputed the idea that it was a documentary at all. "Some people were annoyed by it," Barnard admits. "They felt cheated in some way."

"Documentary," says the dictionary. "Noun. Based on or recreating an actual event, era, life story, that purports to be factually accurate and contains no fictional elements." Small wonder the jury were split. This ironclad definition leaves no room for the likes of The Arbor, with its lip-synching actors, and no room for the recent Alamar, a staged documentary about a Mexican fisherman and his son that nonetheless manages to be tender, touching and true. Presumably it also locks the door on Ari Folman's electrifying animated Waltz With Bashir; a film that was all about false memory and that opened, memorably, with a dream sequence involving a pack of devil dogs. Errol Morris, whose groundbreaking The Thin Blue Line, from 1988, lays on a buffet of dodgy eye-witnesses and corrupted reconstructions, is likewise barred from the party.

It's simply that the further back in film history one goes, the more limiting – if not outright redundant – the dictionary definition turns out to be. The pioneering early documentaries of John Grierson were heavily and openly scripted. Robert Flaherty's 1922 classic Nanook of the North, while embraced as an authentic window on the Inuit lifestyle, was actually staged for the cameras. In fact, it was only with the arrival of cinema vérité in the 1960s that the documentary and the dictionary begins to dovetail. Our concept of the genre as representing a pure and overriding truth is drawn straight from the films of Fred Wiseman, Barbara Kopple and the Maysles brothers, with their handheld cameras, mobile mics and freewheeling sense of life on the run.

Yet were these pictures really the embodiment of purity? Isn't there something doubly suspect about a cinematic technique that pretends it is not a technique at all? "What we are seeing now is a crop of cinema releases that are all about the fluctuation between what's staged and what's real," says Stella Bruzzi, author of the book New Documentary. "And I think those are more trustworthy than films that try to represent truth in an uncomplicated way. The 1960s vérité films that purported to have no authorial intervention are actually much more problematic. I have less of an issue with films that reveal the dialogue between the film-maker and the audience."

Is there a downside in all of this? Bruzzi admits that there may a potential risk of frivolity; the danger that directors will become so wrapped up in the form that they neglect the content. Maybe it will mean less films that make a big statement about the world we live in, and that aren't scared to nail their colours to the mast. Overall, however, she sees it as a healthy development.

"Audiences aren't dumb," she says. "They know that people act differently when you put them in front of a camera, and that just because something is being performed doesn't automatically make it fake. And a technique that distances us from a film needn't make us distrust the film; it just invites us to view it in a different way. Besides, I think we rather enjoy playing those games with a film."

Meanwhile, I find myself thinking about I'm Still Here, Casey Affleck's movie about Joaquin Phoenix, which premiered in Venice last month. Notoriously, I'm Still Here came billed as an access-all-areas documentary, but the film's wild, extrovert stylings set alarm bells ringing. Two weeks ago, Affleck admitted the entire thing was staged; a bravura piece of performance art. And that, by rights, should be the end of that. Except I still find myself wondering if there remains a kernel of truth in there, and whether I'm Still Here might qualify as another of Bruzzi's fluctuations: a film about a troubled, disillusioned actor who decides to play the part of a troubled, disillusioned actor. Think about these things too long and your brain begins to melt.

I turn for advice to Kevin Macdonald. The director has yet to see I'm Still Here, but has a theory on it just the same. "My take on that, knowing some of the people involved, is that the decision [to say it was staged] was made in retrospect," he says. "In that it was an expedient justification to tell everyone it's a fraud. Whereas, on balance, it's probably not."

Macdonald, incidentally, began his career in documentaries. He won an Oscar for One Day in September and a Bafta for Touching the Void, before branching into fiction with The Last King of Scotland and State of Play. He's back in documentary mode when I call, sifting footage for his latest project, Life in a Day. Earlier this year, Macdonald invited the public to shoot a video snapshot of their life on a single day (24 July 2010) and post the footage on YouTube. He now has 80,000 films to view, judge and discard and plans to bring the survivors in at a final running time of under 90 minutes.

Some of the submitted films are not quite what they seem. "We're seeing stuff that is purely faked," he tells me. "Someone in Windhoek faked themselves being mugged. I was watching the film and thinking, 'This doesn't feel right,' and then at the end you see that the mugger is laughing, and has somehow been caught on camera. So I find myself constantly arguing with my editor about what's real and what's not." He cannot be entirely certain that some of these forgeries won't make it through to the final cut.

I like Macdonald because he says stuff I agree with and yet puts it with more elegance and authority than I am able to muster. In Macdonald's view, all documentaries are essentially fictions because they are about organising the chaos of ordinary life; about selecting material and putting that in the service of telling a story. He says film-makers shape the world in their own image. But isn't that what we all do anyway?

"In some ways, making documentaries is like being a journalist," he says. "You interview people and then use the bits you want to use as opposed to the bits they want you to use. I've fallen out very badly with some of the subjects I've interviewed, because they see their lives a certain way; to step into a cinema and see your life depicted in another way can come as a terrible shock."

Like Bruzzi, Macdonald remains broadly supportive of the new strain of documentary, those slippery enterprises that confound the dictionary, employ unreliable witnesses and hazardous fictions, and invite the viewer to play the role of co-conspirator or fellow investigator. He reveres Errol Morris and cites The Thin Blue Line as his masterpiece, even if the film's use of dramatic reconstructions made it ineligible for the best documentary Oscar. "The same thing happened with Touching the Void [which also used reconstructions]," Macdonald says. "But now I think that anything goes. Literally anything. And that's good, so long as the technique tells you something you didn't already know. The only obligation you have as a film-maker is to tell your version of the truth and to use your film to illuminate reality." He laughs. "Whatever that means."